As electronic packaging density increases and dissipated power increases to achieve higher levels of electronic performance, the need for efficient thermal transport within electronic assemblies having printed circuit boards is increasing. Brute force heat transfer techniques involving forced air, active liquid cooling, and similar heat transport mechanisms have been used to transport heat from sensitive electronic components to heat sinks or similar heat spreading devices. Some heat transfer systems use composite structures, for example, annealed pyrolytic graphite (APG) embedded within metallic skins, or use heat pipes that are physically connected to spreader plates by solder, epoxy, or clamps.
These heat transfer systems have benefits and shortcomings depending on the application and environment. In the case of APG composites, in-plane conductivities are on the order of approximately 800-1000 W/m-K at end of life (EOL), but have much higher values at the beginning of life (BOL). This degradation over time is caused, for example, by thermal cycling. Through-plane conductivity is also a concern for APG composites because graphite is orthotropic, and its through-plane conductivity is much lower because of the orientation of in-plane graphite fibers. Despite this in-plane conductivity being six times that of aluminum and two and a half times that of copper, this conductivity is still inferior to that of a typical water-filled copper heat pipe having greater than 10,000 W/m-K in its vapor space, or about ten times that of graphite.
Most heat pipe applications are received in hemispherical grooves and then flattened for direct contact with high heat generating components. In an active heat transfer system, a condenser end of the heat pipe may terminate to permit heat removal, often via fan convection. This type of active heat dissipation may provide good heat transport, but dedicated heat spreaders or heat sinks are required to reduce thermal gradients and improve the conductive transport between the heat sources and heat sink. This technique, however, is not always practical. The heat pipes are exposed to the elements leading to corrosion and often require complex geometries. Other heat pipe designs require clamps, which can introduce undesirable risks or complexity due to heat pipe deformation with respect to clamp load, integration difficulty, and overall design repeatability. These issues impact performance and reliability of the electronic assembly and their integration to printed circuit boards and associated components.